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Old 03-25-2021, 04:43 PM
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4.8is fuel system blues

Hey all! I have been chasing a fuel related issue for several weeks now and it is driving me nuts! It all started when my 2005 X5 4.8is died on the freeway and would not restart. Prior to this happening I had some strange things happening. For one I would hear this strange "whiney" sound coming from the driver side rear passenger seat which was likely the fuel transfer jet pump making the noise. The noise would get higher in pitch and start to fade away after 10sec and then disappear. Occasionally the engine would "stumble" at idle and almost die but would regain itself after a second. And when driving the sometime the acceleration would be poor and even slightly hesitate in first gear but the after a while would magically go away and have great acceleration. So I suspected the fuel pump was bad and sure enough it was; put 12V directly to the fuel pump and motor did not run. After a replacing the fuel pump, it pretty much started right up. After driving a couple of times the whiney sound returned although not as often and also the "stumbles". And I thought, hmm must be the fuel filter and pressure regulator since I did not replace. So then I replaced the fuel filter. Ran the car again and did not change much, still have occasional whiney sound, randomly poor acceleration, and although the car does not stumble, the tone of the idle deepens as if having fuel flow problems but then improves after a while. One thing to note, the car loses fuel pressure so when I go to restart the engine, it does not start right away. And I took off the fuel hose to see if any backpressure after running, and barely any fuel trickled out; I noticed a ton of gas came out for other people on YouTube who replaced the fuel pump and disconnected the fuel line. So now I am at a loss! It almost seemed like it is the fuel pressure regulator but I just changed the fuel filter which has the built in pressure regulator. Could it be a leaking injector? And this is a stretch but could the MAF be defective and sending the wrong fuel demand? Scratching my head...
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Old 03-25-2021, 04:55 PM
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This sounds like a syphon jet issue. Just do a search on this site there is a ton of related info.
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Old 03-26-2021, 03:55 PM
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Quote:
Originally Posted by 80stech View Post
This sounds like a syphon jet issue. Just do a search on this site there is a ton of related info.
Thanks, so the siphon jet would actually cause performance issues? Will try searching.
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Old 03-26-2021, 04:27 PM
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Yep, but I guess it would be more accurate to say "syphon jet plumbing."
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Old 03-26-2021, 09:04 PM
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The siphon jet pump really only matters when you are below about a third of a tank (guess on the exact number there). So if you keep your tank filled at half way or higher, and you still have problems, that should rule out the siphon jet being the only problem you have.

If keeping the gas tank > 1/3 full means the problem goes away, then it reappears if you let it get below that, that would be a very simple and conclusive test that the siphon jet pump is faulty. Andrew Wynn has a great thread on here somewhere with photos and graphics to show you how the pumps and fuel tank work, which should make sense of this.

Here's a good thread on the siphon jet repair, with links to that AndrewWynn thread in post #1:
https://xoutpost.com/bmw-sav-forums/...flon-wrap.html

What brand of fuel pump did you use when you replaced yours? Pierburg was the original manufacturer, bought out by TI Automotive, so either of those are good. The loss of fuel pressure after shutting down may be an important clue. I think the pump has a check valve in it, so loss of pressure may indicate a problem with it holding pressure.
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Last edited by oldskewel; 03-26-2021 at 09:15 PM.
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Old 03-26-2021, 09:27 PM
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Engine running fuel pressure is determined by the syphon jet relief valve so a leaking syphon jet o-ring (for example) could result in up to 8 PSI drop (down to fuel pressure reg setting) in engine running fuel pressure, AND explain the noise from the left side of the fuel tank.
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Old 04-03-2021, 01:57 PM
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Thanks 80s Tech! That is exactly what I think the issue is. I am sure that the noise is coming from the very top of the tank where the pressure release valve is. And I was under the impression that the fuel pump is before the fuel filter/pressure regulator so the residual fuel pressure would be held between the pressure regulator of the fuel filter and the pressure relief valve on the siphon jet assembly. Since I recently replaced the fuel filter/regulator, the only other pressure point would have to be the fault relief valve.
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Old 04-03-2021, 02:34 PM
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You have the right idea but the pressure relief valve is in the fuel pump well on the right side of the tank. The valve itself is probably not the fault and it's more likely the O-rings/seal, on the left side that you say the noise is coming from, which feeds the syphon jets. If you have a look at the fuel diagram in the Bentley manual (or posted here) and pull the left side apart it will start to make more sense. There are a lot of posts/threads about this on the forum as well.
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